Perhaps the two most intense and ongoing debates in the field of muscle strength and size developmental training are: (1) isometric vs. isotonic contraction, and, (2) free weight vs. machine isotonic contraction.
Isometric (contraction without movement) exercise will produce the most strength increase with the shortest training time. However, the strength increase is specific for a very short and limited span within the full potential range of the muscle.
Obviously, such a strength increase is of little practical value in sports or other activities. If the muscle is isometrically exercised at many positions throughout its range of motion, this limitation can be circumvented. However, isometric exercise shuts off the blood supply to the muscle. Therefore, isometrically developed strength is not accompanied by increased muscle capillarization, increased local anaerobic endurance, or with maximum muscle size increases. All of these additional benefits are normally associated with intense isotonic strength training. Also, isometric exercise does not promote the development of increased muscle flexibility (and the subsequent reduced risk of sports injury) since the muscle is not worked over its full range of motion. Since the resistance utilized is isometric training is immobile, increased muscle coordination and control is also unaffected.
In addition to increased strength, all of these additional benefits are provided by both free weights and machine isotonic strength training except for increased muscle coordination and control. This benefit is provided only by the free weights, because balance and control are required for their use as opposed to machines containing weights or other resistance restricted to an exact limited plane of motion. This "limitation" does, however, produce safety from the dangers of falling weights.
Many machine weight systems intentionally allow a much greater isolation of individual muscles as opposed to free weights or some other machines. Although this may be of great benefit in certain extremely specific instances such as injury rehabilitation training, or for a top level bodybuilder trying to provide extra work to only certain slower growing muscles, the other claimed advantages are dubious as best. In fact, in the "real world" of everyday activities and sports, muscles almost always work together as large groups. Potential overdevelopment of a specific muscle by isolation training may hinder useful interactions within the group.
Some machines have systems of cams, levers, etc., which allow the exercising muscles to experience constant resistance throughout the full range of motion of the exercise. This should allow for more muscle fibers to be stimulated. Free weights provide varying resistance throughout the movement as the mechanical advantage is greater at some points in the motion than in others. For example, in the fundamental weight training exercise, known as the bench press, the movement begins with the subject prone on a bench with a barbell positioned on the chest. The weight is pushed upwardly until the arms are fully extended with the elbows locked in a completely unbent position above the chest.
Due to the change in mechanical advantages, the same weight provides much more resistance at, for example, a position halfway between the bottom and top as opposed to a position 9/10 of the way from the bottom and thus with the arms almost straight. Although the bench press utilizes chiefly the chest, shoulder, and tricep muscles, the individual muscle fibers used are not all the same at the 1/10, 5/10, and 9/10 positions. Some fibers potentially used at the 8/10 and 9/10 positions will never be used or developed because the resistance is insufficient to require their use. If enough weight was provided to provide sufficient development for these fibers, the subject exercising would never be able to move the bar off of the chest.
A modified bench press on certain machines will provide equal resistance at all positions. However, the loss of a need to balance or control the weight can detract from the "real world" usefulness of the strength developed.